The events are as follows which spread the use of glass ionomer cement (abbreviated as “GIC” in this specification).    (1) Phosphate cement before 1967. This phosphate cement has features of engagement strength with dentin and coalescence strength. In addition to this,    (2) carboxylate cement, which D. C. Smith developed in 1968. This carboxyl cement has a feature of adhesive strength to dentin. However, it was seen that carboxyl cement had disadvantages such as lack of mechanical strength, and large thermal expansion difference between dentin and this cement. For this reason, in order to solve the disadvantages,    (3) GIC was introduced by A. D. Wilson in 1971.
This GIC has advantages and features that cannot be obtained by other materials. The advantages and features are as follows.    (1) Low irritation to and biological affinity for dental pulp.    (2) Secondary dental caries suppression effect and antibacterial activity by release and recharge of fluorine ion.    (3) High adhesiveness to dentin and metal (chemical adhesion).    (4) Elimination of tooth surface treatment (acid etching) in use by this high adhesiveness.    (5) Close thermal expansion coefficient to dentin.    (6) GIC is translucent after cured. This provides an excellent aesthetic feature and the like.
The GIC in the introduction was chemically-curing type (or chemically-polymerized type) GIC that was used by reacting silicate cement (for fore tooth restorative filling) powder or glass powder with polymeric acid principally composed of acid such as polycarboxylic acid in water-existing conditions. It was said that silicate cement had high dental caries protection from long ago.
However, it was seen that the GIC had disadvantages of embrittlement caused by its water sensitivity (high solubility) in the early curing stage, low bending strength (fragility) and the like. For this reason, it was required to prepare the GIC with careful attention in clinical use of prosthetic adhesion, filling and the like.
Afterward, in order to increase mechanical strength, resin group GIC was developed which was obtained by adding and mixing a resin component into the aforementioned chemically-curing type GIC. Furthermore, light-curing type (photo-polymerized type) GIC became commercially practical which was obtained by employing a photo-polymerized type resin and a photo polymerization catalyst and was quickly cured. Today, among various types of commercially available GIC such as chemically-curing type GIC and resin group GIC, the light-curing type GIC is widely used and is in the mainstream.
However, in this light-curing type GIC, the aforementioned original advantages decrease. The following disadvantages are known.    (1) Added resin causes secondary caries and irritation to dental pulp, and reduces biological affinity for dental pulp.    (2) The slow-release amount of fluoride decreases.    (3) Tooth surface treatment (acid etching) is required in adhesion.    (4) Since a light irradiation device is required, light-curing type GIC is not convenient and cannot be used in developing countries, and the like.
In order to solve the disadvantages of the resin group GIC and light-curing type GIC, technology is under development to improve the mechanical strength of GIC without using resins. For example, technology is known which adds and mixes apatite fiber into conventional glass powder for GIC to improve the strength of chemically-curing type GIC (see Japanese Patent Laid-Open Publication TOKUKAI No. 2001-354509; Dental Materials Journal, Vol. 22 (No. 2), 126-136, 2003, and Biomaterials, Vol. 24, 3787-3794, 2003). Also, technology is developed which adds apatite nanoparticles of 200 nm or less.
However, in the case of the aforementioned chemically-curing type GIC, the reinforcement (improvement) by apatite addition and mixture is about 30% in mechanical strength. This is about 40% lower as compared with the reinforcement (improvement) by light-curing type GIC. For this reason, light-curing type GIC is widely used and is in the mainstream today. The aforementioned chemically-curing type GIC cannot be evaluated as a substitute for light-curing type GIC.
Also, in the case of GIC of US patent publication US 2005/0260269, hard, strong and fine apatite crystalline particles are added which are nanoparticles that are chemically formed by solution deposition (wet process). This GIC has a disadvantage in that curing time cannot be shortened to the extent of actually available use. In addition to this, though crystals of apatite that are hard nanoparticle are used as a filler (reinforcing member), the crystals of apatite cannot reinforce the mechanical strength of GIC in a cured state.